Specimen 1961-PC-2/7B-11-4A. The picture shows the dorsal (left) and ventral (right) views of the abdomen of a female Cimex pilosellus recovered from Paisley Caves, and is approximately 9,400 years old. The thorax and the head (as well as all appendages) are missing from this specimen. The scale bar at the bottom represents one millimeter.

MIAMI(AFP).- The oldest known remains of bed bugs have been found in a US cave, but these ancient critters did not feed on human blood like the modern insects, researchers said Tuesday.

Rather, the species found in the Paisley Five Mile Point Caves in southern Oregon -- Cimex pilosellus, Cimex latipennis, and Cimex antennatus -- are all parasites of bats, said the study in the Entomological Society of America's Journal of Medical Entomology.

They range in age from 5,100 to 11,000 years old.

Previously, the oldest known bed bugs -- or cimicids -- were 3,500 years old, and were uncovered in Egypt in 1999.

The US specimens were identified as relatives of the present-day bed bug, Cimex lectularius, but are "not the bed bug we all know and love from hotel rooms," said co-author Martin Adams, who heads a company called Paleoinsect Research which specialized in identifying old bugs.

The two bed bug species that parasitize humans -- Cimex lectularius and Cimex hemipterus -- are believed to have adapted to human hosts when people shared caves with bats in Europe, Asia, and Africa thousands of years ago.

Adams said it is unclear why the bed bugs in Oregon did not make that leap.

"Were the cimicid populations too small to establish themselves outside the caves, or were the host populations too small?" Adams said.

"Given that Paisley Caves was only a seasonal occupation area for human hunter-gatherers, did the humans move around too much, or were the bugs not able to withstand the environment outside the caves for very long?

"Or, were there other constraints involved? I'm working on these last few archaeological questions right now."

Adams co-authored the study along with Dennis Jenkins of the Museum of Natural and Cultural History at the University of Oregon.